1. Field of the Invention
The present invention relates to film magazines, and more particularly, to improvements in such magazines for reducing the force required to initiate the unwinding of film from a film supply roll wound within the magazine.
2. Description of the Prior Art
In film magazines of the general type to which the present invention relates, a film supply roll is wound on a core rotatably supported in a film supply compartment. When initially wound on the core, the inherent resiliency of the film tends to cause the film to try to return to a straight configuration, and when the magazine is subjected to shock and vibration, the film tends to straighten within the magazine. This causes the film convolutions to expand, commonly referred to as clock-springing, until the outermost convolution is in intimate frictional contact with the magazine's inner wall surface.
The frictional engagement of the film against the magazine's walls resists initial unwinding rotation of the roll when a camera's film transport mechanism applied tension to the leading end of the film. The resistance is reduced when sufficient film has been unwound to relieve the friction, whereupon the remainder of the film can be unwound by the application of considerably less film tension.
Previously known cameras employing film magazines with which the above-described phenomenon is encountered have been adapted to overcome the initially high resistance of the film to unwinding simply by the application of sufficient film tension to overcome the frictional engagement between the magazine's walls and the periphery of the supply roll. However, this approach to the problem entails the possibility of tearing or otherwise damaging the film through the application of excessive tension. Also, a camera incorporating such a relatively powerful film transport mechanism must be provided with a correspondingly strong metering system capable of intermittently inhibiting movement of the transport mechanism as successive film exposure areas are advanced into exposure position. Futhermore, and perhaps most importantly, in a camera provided with a film transport mechanism powered by an electric or spring-actuated motor, the extra torque required to initiate the film transport operation dictates that the drive motor be correspondingly larger and more powerful than is necessary to transport the film after the initial unwinding resistance has been overcome.
The problem is somewhat alleviated if after spooling, the loaded magazines with tightly wound film rolls are allowed to sit without being subjected to shocks and vibrations. As the film ages, its inherent plasticity causes it to set, and its clock-springing tendency diminishes. However, there remains some risk that subsequent jarring during shipping and handling will cause the film convolutions to expand and the outermost convolution to frictionally engage the magazine walls.